The present invention relates to a sheet feeding apparatus and more specifically, to a sheet feeding apparatus for an image forming apparatus, which feeds a sheet one by one from a stack of sheets to an image forming unit of an image forming apparatus.
In the image forming apparatus such as a copying machine, there is provided a sheet feeding apparatus which includes a feeding cassette, a sheet forwarding unit for forwarding a sheet one by one from the feeding cassette to the image forming unit, and a sheet detection gate for detecting whether the sheet is actually forwarded from the sheet forwarding unit or not. The sheet forwarding unit includes a feeding roller abutting on an uppermost sheet in the feeding cassette, and a pair of upper and lower separating rollers.
According to conventional devices like the above sheet feeding apparatus, when the print key of the copying machine is pressed, the feeding roller abutting on a selected sheet starts to rotate at a predetermined timing. Then, a sheet separated by the separating rollers passes through a sheet detection gate and is then supplied to the image forming unit.
In the above example, if the detection gate does not detect the sheet even after a predetermined time has elapsed from the start of the forwarding operation, it is determined that sheet-feeding defect has generated in the sheet forwarding unit due to a paper jam. Then this fact is displayed at the operation panel, whereupon tile operation of tile machine is halted. In order to start the operation again, the jammed sheet is taken out, the sheets in the feeding cassette are aligned, and the reset switch is pressed.
According to the above conventional structure, the operation has to be restarted each time, when the detection gate does not detect tile sheet within a predetermined time, resulting in a determination that the paper is jammed thus halting operation.
However, feeding defects are not only caused by the paper jam but may also be caused by wear of the feeding roller. When the feeding defect is caused by the wear of the feeding roller, the sheet can be supplied in many cases simply by repeating the feeding operations several times.
The above conventional structure, however, can not determine the cause of the feeding defect. Consequently, a feeding defect detected even once is determined to be a paper jam and hence the machine is immediately stopped. Therefore, conventional machines may be often unnecessarily halted due to feeding defects caused by the wear of the feeding roller, although these defects would easily be resolved by just repeating the feeding operation.
Japanese Patent Publication No. 43620/1993 discloses a jam-check means for a sheet transport apparatus which includes a first counter which counts the number of times sheets are transported, and second counters which each count the number of jams occurring at certain jam locations. This jam-check means sends out a warning when a number of jam occurrences exceeds a predetermined value in any of the jam locations. Thus, discovery of the problem area becomes much easier. However, it does not distinguish between supply forwarding failures due to jamming and those due to a worn supply roller.